The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
In some vehicles, a high voltage (HV) battery may be used to provide power to an inverter and electric motor, which drives one or more wheels of the vehicle. The HV battery may operate at a high voltage such as 250V to 1000V DC. Accessories and other vehicle systems may be powered by a low voltage (LV) system such as a 12V DC battery. For safety reasons, the HV battery and all circuits connected to the HV battery should be Galvanically isolated from the vehicle chassis as well as other systems of the vehicle.
For example only, there can be physical and parasitic capacitive coupling from the inverter to the chassis, from the electric motor to the chassis as well as undesirable leakage resistance from the electric motor or HV side of the inverter to the chassis. To ensure safety, isolation of the HV battery is monitored. When an isolation problem is detected, the monitoring system may alert the vehicle operator and/or take action.
Isolation monitoring circuits for the HV battery may use DC or AC monitoring methods. In DC monitoring methods, the HV battery (or HV electrical power source) pushes current through the leakage impedance to the chassis. The resultant current or voltage is measured across another high value resistance. In AC monitoring methods, a small AC current is injected into the HV battery and the resultant AC voltage or current is measured.
In the DC monitoring method, direct and indirect connection methods can be used. In the indirect approach, one or more optical switches are used to indirectly connect the HV rails through a high impedance to the chassis. The optical switches tend to be expensive. Temperature and aging variation of the optical switches may create potentially erroneous measurements that may be false negatives thus creating the possibility of shock hazards. With DC isolation fault detection methods, fault detection generally cannot be performed when the HV battery is removed.
In the direct approach, the potential of the HV battery is also required. Fault detection is also not possible when the HV battery is removed. Both variants may have issues with large parasitic and system filter capacitors as well as operating reliably in noisy environments. Additionally this approach creates an intentional leakage path from the HV battery to the chassis to complete the measurement circuit, which is contrary to the goal of isolation.
Advantages of the AC monitoring method relative to the DC monitoring method include the ability to utilize a capacitor to inject the signal, which eliminates the need for the optical switches. Another advantage is the ability to run isolation tests without having the HV battery installed. However, in many cases, reactive shunt AC impedance is much lower than the fault impedance itself, which makes reliable isolation monitoring measurements difficult.